1. Field of the Invention
The present invention relates to an exhaust gas purification system for a diesel engine with a catalyst installed in an exhaust conduit of the engine and constructed of zeolite carrying at least one kind of metal selected from the group consisting of transition metals and noble metals to reduce nitrogen oxides (NOx) included in exhaust gas from the engine under oxidizing gas conditions and in the presence of hydrocarbons (hereinafter, a lean NOx catalyst).
2. Description of the Prior Art
A lean NOx catalyst is known from, for example, Japanese Patent Publication No. HEI 1-130735 and Japanese Patent Publication No. HEI 1-135541.
The lean NOx catalyst can show a high NOx purification rate only in a certain temperature range (for example, 400.degree. C.-550.degree. C.). More particularly, if the catalyst temperature is higher than an upper limit of the temperature range, hydrocarbons (HC), which are necessary for reduction of NOx, included in the exhaust gas are oxidized, so that the amount of HC will be insufficient. In contrast, if the catalyst temperature is lower than a lower limit of the temperature range, the activity of the lean NOx catalyst is lost, so that the NOx purification rate of the lean NOx catalyst is decreased.
Since a diesel engine is operated at lean air-fuel ratios, that is, under oxidizing gas conditions, combination of the diesel engine and the lean NOx catalyst should be desirable. However, in a direct fuel injection-type diesel engine where fuel is directly injected into a cylinder of the engine, since the exhaust gas temperature increases in proportion to an increase in the fuel injection amount, there is only a narrow torque range where the lean NOx catalyst can show a high enough NOx purification rate to be usable. As a result, equipping a diesel engine with a lean NOx catalyst cannot be expected to produce a great improvement of the NOx purification rate.